Proto-Indo-European root meaning "to let go, slacken."
It is the hypothetical source of/evidence for its existence is provided by: Greek ledein "to be weary;" Latin lenis "mild, gentle, calm," lassus "faint, weary;" Lithuanian lėnas "quiet, tranquil, tame, slow," leisti "to let, to let loose;" Old Church Slavonic lena "lazy," Old English læt "sluggish, slow," lætan "to leave behind."
"place or opening by which anything is let out or escapes," mid-13c., "a river mouth," from out- + let (v.). Electrical wiring sense, "socket that connects a device to an electricity supply," is attested from 1892. The commercial sense of "a market for the sale of any product" is by 1889; that of "a retail store disposing of a manufacturer's products" is attested by 1933. Figurative sense "means of relief or discharge" is from 1620s.
1786, from verbal phrase (attested by 1711) in reference to one who "ruins" the "fun;" see spoil (v.) + sport (n.). Compare Chaucer's letgame "hinderer of pleasure" (late 14c.), from obsolete verb let (Middle English letten) "hinder, prevent, stop" (see let (n.)). Another old word for it was addle-plot "person who spoils any amusement" (1690s; see addle).